bug#33195: 27.0.50; user-login-name has no group-login-name

[Jules Tamagnan <jtamagnan@gmail.com>]

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I noticed that I had marked GID as optional in the manual, below is a
patch with that fixed.


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From 5c67d0177ba14428e6e3f5acdaf34c2be7ee93be Mon Sep 17 00:00:00 2001
From: Jules Tamagnan <jtamagnan@gmail.com>
Date: Tue, 30 Oct 2018 10:22:03 -0700
Subject: [PATCH] src/editfns.c (group-name): New function.

---
 doc/lispref/os.texi       |  5 +++++
 etc/NEWS                  |  3 +++
 src/editfns.c             | 16 ++++++++++++++++
 test/src/editfns-tests.el |  7 +++++++
 4 files changed, 31 insertions(+)

diff --git a/doc/lispref/os.texi b/doc/lispref/os.texi
index cb33757..6d1b3f3 100644
--- a/doc/lispref/os.texi
+++ b/doc/lispref/os.texi
@@ -1230,6 +1230,11 @@ User Identification
 return value is @code{nil}.
 @end defun
 
+@defun user-login-name gid
+This runction returns the group name that corresponds to @var{gid},
+or @code{nil} if there is no such group.
+@end defun
+
 
 @node Time of Day
 @section Time of Day
diff --git a/etc/NEWS b/etc/NEWS
index 226ae1e..a0bcbb5 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1228,6 +1228,9 @@ where there's no better alternative.  We believe that the incorrect
 uses of this function all but disappeared by now, so we are
 un-obsoleting it.
 
++++
+** New function 'group-name' returns a group name based on a group-GID
+
 \f
 * Changes in Emacs 27.1 on Non-Free Operating Systems
 
diff --git a/src/editfns.c b/src/editfns.c
index e995b38..15a0fa7 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -1143,6 +1143,21 @@ of the user with that uid, or nil if there is no such user.  */)
   return (pw ? build_string (pw->pw_name) : Qnil);
 }
 
+DEFUN ("group-name", Fgroup_name, Sgroup_name, 1, 1, 0,
+       doc: /* If argument GID is an integer or a float, return the login name
+of the group with that gid, or nil if there is no such GID.  */)
+  (Lisp_Object gid)
+{
+  struct group *gr;
+  gid_t id;
+
+  CONS_TO_INTEGER (gid, gid_t, id);
+  block_input ();
+  gr = getgrgid (id);
+  unblock_input ();
+  return (gr ? build_string (gr->gr_name) : Qnil);
+}
+
 DEFUN ("user-real-login-name", Fuser_real_login_name, Suser_real_login_name,
        0, 0, 0,
        doc: /* Return the name of the user's real uid, as a string.
@@ -4487,6 +4502,7 @@ it to be non-nil.  */);
   defsubr (&Sinsert_byte);
 
   defsubr (&Suser_login_name);
+  defsubr (&Sgroup_name);
   defsubr (&Suser_real_login_name);
   defsubr (&Suser_uid);
   defsubr (&Suser_real_uid);
diff --git a/test/src/editfns-tests.el b/test/src/editfns-tests.el
index 17b2c51..6ee0ab0 100644
--- a/test/src/editfns-tests.el
+++ b/test/src/editfns-tests.el
@@ -351,4 +351,11 @@ transpose-test-get-byte-positions
     (should (equal (format "%-#50.40x" v3)
                    "-0x000000003ffffffffffffffe000000000000000        "))))
 
+(ert-deftest group-name ()
+  (let ((list `((0 . "root")
+                (1000 . ,(user-login-name 1000))
+                (1212345 . nil))))
+    (dolist (test list)
+      (should (equal (group-name (car test)) (cdr test))))))
+
 ;;; editfns-tests.el ends here
-- 
2.7.4